Undertale biggest sprite inconsistency

Something that really grinds my gears is the fact that when Undyne walks left on Undertale, her sprite lacks her eyepatch and we can see her eye. Despite knowing she lacks an eye there. It's something that can go unnoticed in your first playthrough but once you noticed it, it really makes you thinking: WHY TOBY??? WHY UNDYNE'S EYE IS NOT MISSING???